Fluoropolymers, i.e. polymers having a fluorinated backbone, have been long known and have been used in a variety of applications because of several desirable properties such as heat resistance, chemical resistance, weatherability, UV-stability etc. The various fluoropolymers are for example described in “Modern Fluoropolymers”, edited by John Scheirs, Wiley Science 1997. The fluoropolymers may have a partially fluorinated backbone, generally at least 40% by weight fluorinated, or a fully fluorinated backbone. Particular examples of fluoropolymers include polytetrafluoroethylene (PTFE), copolymers of tetrafluoroethylene (TFE) and hexafluoropropylene (HFP) (FEP polymers), perfluoroalkoxy copolymers (PFA), ethylene-tetrafluoroethylene (ETFE) copolymers, terpolymers of tetrafluoroethylene, hexafluoropropylene and vinylidene fluoride (THV) and polyvinylidene fluoride polymers (PVDF).
A frequently used method for producing fluoropolymers involves aqueous emulsion polymerization of one or more fluorinated monomers resulting in an aqueous dispersion of the fluoropolymer. The aqueous emulsion polymerization of fluorinated monomers generally involves the use of a fluorinated surfactant. Frequently used fluorinated surfactants include perfluorooctanoic acids and salts thereof, in particular ammonium perfluorooctanoic acid. Further fluorinated surfactants used include perfluoropolyether surfactants such as disclosed in EP 1059342, EP 712882, EP 752432, EP 816397, U.S. Pat. No. 6,025,307, 6,103,843 and 6,126,849. Still further surfactants that have been used are disclosed in U.S. Pat. No. 5,229,480, 5,763,552, 5,688,884, 5,700,859, 5,804,650, 5,895,799, WO 00/22002 and WO 00/71590.
Perfluorocarboxylic acids (PFCA) are the preferred emulsifiers for making fluorinated polymers, e.g. perfluorinated polymers like PTFE, FEP, PFA, perfluorinated elastomers, and others. Especially perfluorooctanoic acid (PFOA) in form of its salts (e.g.ammonium salt, APFO) is widely used. But, APFO and other fluorinated surfactants, in particular perfluorinated surfactants have raised environmental concerns. Another important aspect is the fact that these surfactants are expensive materials and any losses thereof from the production process should be minimized. Until now, these emulsifiers, especially APFO are indispensable because they do not display chain transfer properties. So PFOA or APFO respectively are just a prominent example for a whole class of fluorinated surfactants, in particular fluorinated surfactants with carboxylic acid groups.
The fluoropolymers may be used to coat substrates to provide desirable properties thereto such as for example chemical resistance, weatherability, water- and oil repellency etc. For example aqueous dispersions of fluoropolymer may be used to coat kitchen ware, to impregnate fabric or textile e.g. glass fabric, to coat paper or polymeric substrates. For sake of economy and convenience, the fluoropolymer dispersions will typically have between 35% by weight and 70% by weight of fluoropolymer solids, which is typically attained using an upconcentration process. Alternatively, for some applications, the fluoropolymers are provided in granular or powder form. To obtain fluoropolymer granulate or powder, the fluoropolymer is typically coagulated and the resulting coagulate may be washed with water one or more times to obtain a desired level of purity.
During the production of fluoropolymers to their final commercial form, waste water streams are created that contain fluorinated surfactant. For example, waste water streams may result from upconcentration of the dispersion, cleaning of the polymerization vessel and equipment, coagulation of the dispersion and washing to obtain fluoropolymer granulate or powder. Additionally, waste water containing fluorinated surfactant may result during application of the fluoropolymers. Frequently, the waste water streams not only contain fluorinated surfactant but also other components such as a small amount of fluoropolymer particles.
Several methods for the removal of PFCAs from aqueous media are known. For example, a method employing reverse osmosis is described in WO 02/139593. A combined process of extracting PFCA from aqueous solutions at low pH levels using chlorinated hydrocarbons and contacting the organic layer with alumina to recover the PFCA is described in EP 194692 and EP 194691. DE 2407834 discloses the use of silica gel to separate PFCAs from aqueous solutions.
Treatment of PFCA contaminated water can be done by applying reverse osmosis followed by an active carbon bed absorption including the regeneration thereof with ethanol as described by G. A. Bystrov et al, Plasticheskie Massy, (1990), (4), 75-8 (CA 113, 11571). As reported by the Russian Authors, the PFCA contaminated water (40-4000 mg of PFCA per liter) is purified by reverse osmosis in an initial step, resulting in water containing less than 20 mg per liter of PFCA. This level can be further reduced in an additional purification step using an active carbon bed. At break through of PFCA, the loaded active carbon bed is regenerated. Although several different methods were tried, the Soxhlet extraction with solvents, especially a ethanol-water mixture, showed the best results. But even in this case only 65% of the absorbed PFCA could be removed. The thus regenerated active carbon showed a decrease of activity in the range of 25-40%. Based on this result it is stated that the active carbon can be reused only 2-3 times before it has to be discarded.
It will generally be desired to recover the fluorinated surfactant from the adsorbent particles such that the expensive fluorinated surfactant can be reused in a polymerization process and the adsorbent particles can be reused in a purification of waste water. While the efficiency of the adsorbent particles may decrease after reuse, it would be desirable to regenerate the adsorbent particles such that they can be reused more frequently before they have to be discarded because of unacceptable low efficiency levels.
A still further method concerns the use of an anion exchange resin to recover PFCAs from fluoropolymer particle containing waste water. Such method has been disclosed in WO 99/62858 and WO 99/62830. According to WO 99/62858, the fluoropolymer particles are removed from the waste water before contacting the waste water with the anion exchange resin.
According to WO 99/62830, a non-ionic surfactant is added to the waste water before contacting the latter with the exchange resin. Thus, in this method the PFCA is bonded to the exchange resin via an anion exchange mechanism but also physical adsorption to the resin particles is believed to take place in the removal process. According to the teaching of these WO applications, the fluorinated surfactant can be recovered from the anion exchange resin by eluting the anion exchange resin with an appropriate regeneration fluid releasing the fluorinated surfactant from the anion exchange resin. A disadvantage of the recovery method for the fluorinated surfactant from an anion exchange resin is that large amounts of regeneration are generally required which adds to the cost of regeneration and is further inconvenient.
It would thus be desirable to find a further process for the recovery of fluorinated acid surfactants, from adsorbent particles loaded with the fluorinated surfactant. Desirably such process is efficient, use minimal amounts of regeneration fluids, preferably is convenient and preferably results in regenerated adsorbent particles that can be reused multiple times.